MIPS: Improve SeqStringSetChar implementation.

Port r16707 (a25d669)

TEST=/test/mjsunit/lithium/SeqStringSetChar.js
BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/23484043

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16720 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: remove getcurrent from stubs.

Port r16710 (c5203f8)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/23710046

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16721 ce2b1a6d-e550-0410-aec6-3dcde31c8c00